Smallest ship I was ever on was 29,000 ton and made in Scotland on the Clyde in 1957. It was my first cruise, and has a special place in heart.
What you have to remember is that what some see as cruising with a choice ie do I go Costa, RCCL, P&O, Celebrity, Princess etc.
They are all owned by Carnival, so they are bulk market with a company board directed towards bulk.
So unless you go to the smaller independants, you will never see smaller ships from these guys unless it's in response to a niche in the market, then its expensive and specialist.